Bump urllib3 from 1.26.2 to 1.26.3

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.2 to 1.26.3.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/1.26.3/CHANGES.rst)
- [Commits](urllib3/urllib3@1.26.2...1.26.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -31,7 +31,7 @@ six==1.15.0
 transaction==3.0.1
 translationstring==1.4
 ujson==4.0.2
-urllib3==1.26.2
+urllib3==1.26.3
 venusian==3.0.0
 waitress==1.4.4
 WebOb==1.8.6